The optical memory technology which involves use of an optical disk medium having a pit pattern as a high-density and large-capacity information storage medium is being put to practical use, with its purposes being expanded to digital audio disks, video disks, text file disks, and further to data file disks. The functions which are necessary for using a finely-focused laser light to successfully perform recording of information onto an optical disk medium and reproduction of information from an optical disk medium in a highly reliable manner are generally classified into: a convergence function of forming a diffraction-limited light spot; a focus control (focusing servo) function for an optical system and a tracking control (tracking servo) function; and a pit signal (information signal) detection function.
In order to further enhance the recording density of an optical disk medium, the numerical aperture NA of an objective lens to be mounted in an optical head is on the increase and the wavelength λ of light from a light source is on the decrease, whereby the spot diameter of light which is converged by the objective lens is being reduced. Moreover, in order to further increase the recording capacity of an optical disk medium, there has been a trend to provide in an optical disk medium a plurality of recording layers for recording information.
For example, according to the CD (Compact Disc) standard, the numerical aperture NA of an objective lens is 0.45 and the wavelength of light from a light source is 780 nm. On the other hand, according to the DVD (Digital Versatile Disc) standard, which is adapted for a higher recording density and a larger capacity, the numerical aperture NA is 0.6 and the light wavelength is 650 nm.
By the way, an aberration which occurs due to a tilt of an optical disk medium with respect to the optical axis increases as the wavelength of light becomes shorter. Therefore, in order to employ a shorter-wavelength light source and still obtain a good light spot, it is effective to reduce the base thickness (i.e., a distance from a light-entering surface of an optical disk medium to a recording layer) in a manner to cancel such an aberration. Thus, the base thickness is 1.2 mm for the CD, but is 0.6 mm for the DVD.
Moreover, according to the BD (Blu-ray Disc) standard, which uses blue laser light, the numerical aperture NA is 0.85, and the base thickness is 0.1 mm.
A spherical aberration which is ascribable to the base thickness protecting a recording layer of an optical disk medium is in proportion with the fourth power of the numerical aperture NA. Therefore, when setting a large numerical aperture NA of 0.85, as in the case of the BD standard, a means for correcting spherical aberration is provided in the optical system.
Conventionally, a technique has been proposed which detects a specific type of aberration from the output signal of an optical pickup device and corrects the aberration. There are transmission-type aberration correcting devices using liquid crystal devices, as well as reflection-type aberration correcting devices which drive thin film mirrors or arrayed micromirrors, as are disclosed in Patent Document 1 and Patent Document 2.
Furthermore, although the market demand for downsizing of electronic devices is only increasing, progresses in semiconductor micromachining technology have facilitated the fabrication of micro structures such as aberration correcting devices. In the recent years, this technology is referred to as the micromachining technology, and there is a great deal of research and development directed to microactuators to which this technology is applied. One instance of an actual product of this is an acceleration sensor of a capacitance detection-type. Another instance is a spatial light modulation apparatus disclosed in Patent Document 3. This has been developed as an image displaying device for a video projector, and is known by the name of DMD (Digital Micromirror Device). Although this microactuator uses electrostatic attraction as its driving method, other examples would be electromagnetic force, thermal stress, optical pulling force, and the like.
A driving method which is used particularly frequently is electrostatic attraction. This method has an advantage in that it is capable of minimizing the driving voltage with a low power consumption, while also realizing a high response speed.
Furthermore, Patent Document 4 discloses a deformable mirror which, on the strength of the fact that electrostatic attraction is utilized as its driving force, realizes a displacement detection function of detecting its own displacement based on changes in capacitance.
Moreover, Non-patent Document 1 discloses a chip on which a temperature sensor, a pressure sensor, and a humidity sensor are collectively mounted. Each of these sensors is designed individually in accordance with the subject of detection, such that a different material is used for each sensor, each sensor being produced on its own.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2000-155979
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2002-288873
[Patent Document 3] Japanese Laid-Open Patent Publication No. 8-334709
[Patent Document 4] Japanese Laid-Open Patent Publication No. 2002-228813
[Non-patent Document 1] “AN ALL-CAPACITIVE SENSING CHIP FOR TEMPERATURE, ABSOLUTE PRESSURE, AND RELATIVE HUMIDITY”, IEEE 12th International Conference on Solid-State Sensors, Actuators and Microsystems, Boston, Mass., June 2003